What animal are you trying to protect? Because fleas live on larger, warm-blooded hosts, the only effective predators would be some creature that the host is disposed to tolerate, like the birds you see on African rhinos in nature documentaries. I don't know of any predators playing that role for dogs and cats. Most flea treatments are based on killing the fleas through chemical or environmental means, like diatomaceous earth dehydrating the fleas and other invertebrates.
Weeds are just plants with enough surplus will to live to withstand normal levels of gardening!--Alexandra Petri
The world's cheapest jedi mind trick: "Aw c'mon, why not read this tiny ad?"